I can't help you with that, specially since I have no knowledge
of igraph library at all.

As instructed on igraph gem readme (which I provide you a link
to) there is a section mentioning how to install the gem once you
have igraph installed, which already provides a link to the library
website where you should be able to download it, compile it and
install it.

If you're not familiar with library compilation, you can try
read about it online.

You can try asking this on specialized support groups like
RailsInstaller or RubyInstaller: